A phase shifting transformer is previously known for controlling the power flow in an ac transmission line. Such PST comprises a tap changer that serially connects or disconnects additional windings of the transformer. By doing so the phasor orientation is controlled. Power is then moved from adjacent phases to a single phase by connections between windings excited by different parts of the magnetic circuit. In a pure phase shifting transformer a voltage in quadrature to the source voltage is injected into the line.
A phase shifting transformer may be used to control the load distribution between parallel lines to increase total power transfer. Advantageous is the phase shifting transformers capability to block parasitic power flow due to phase angle difference in a feeding network. Power may be distributed to customer in a defined way and circulating power flows may be avoided.
The use of a PST is advantageous in that it has relatively low reactive power consumption. There is no risk of a subsynchronous resonance (SSR) and it is powerful also at low current conditions.
The use of a PST however offers a slow control speed. The tap changer has to go through every tap position in a sequential manner. Each tap change is effected in the order of 3-5 seconds. Thus the PST cannot participate in a decisive way in a transient period following a power disturbance. Further frequent tap changing, in particular at high current conditions, increases the need for maintenance.
The tap changer is a mechanical device and thus slow in operation and an object to mechanical wear. It has a maximum regulation voltage range of 150 kV and a maximum number of operating positions of less than 35. The maximum tap voltage is in the order of 4000-5000V/tap and the maximum rated through current is about 3000-4500 A. The maximum power handling capacity is 6000-8000 kVA/tap and there is a short circuit thermal limit. Small voltage steps make many operations.
Another way to control the power flow in an ac transmission line is the use of a static series compensator, especially a Static Synchronous Series Compensator (SSSC). Such SSSC normally contains a Voltage Source Converter (VSC). In a SSSC a voltage in quadrature to the line current is injected into the line.
A SSSC is controllable from full inductive to full capacitive regulation, and vice versa, within a few fundamental frequency cycles and is thus capable of being a powerful control device in the transient period following a power disturbance. In comparison with a tap changer of a PST the SSSC is not maintenance sensitive to frequent control actions. A SSSC is therefore suitable for closed loop control. However, the SSSC has a larger variation in reactive power production/consumption than the PST. In inductive operation the SSSC consumes reactive power and in capacitive operation the SSSC produces reactive power. Furthermore the SSSC is expensive compared to the PST.